Annotation Type Entity

Indicates a persistent entity class. For each entity class, a PrimaryIndex can be used to store and access instances of that class.
Optionally, one or more SecondaryIndex objects may be used to access
entity instances by secondary key.

Entity Subclasses and Superclasses

An entity class may have any number of subclasses and superclasses;
however, none of these may themselves be entity classes (annotated with
Entity).

Entity superclasses (which must be annotated with Persistent, not
Entity) are used to share common definitions among entity classes.
Fields in an entity superclass may be defined as primary or secondary keys.
For example, the following BaseClass defines the primary key for any
number of entity classes, using a single sequence to assign primary key
values that will be unique across all entity classes that use it. The
entity class Pet extends the base class, implicitly defining a
primary index

Entity subclasses (which must be annotated with Persistent, not
Entity) are used to provide polymorphism within a single PrimaryIndex. Instances of the entity class and its subclasses are stored
in the same PrimaryIndex. For example, the entity class Pet
defines a primary index that will contain instances of it and its
subclasses, including Cat which is defined below.

Fields in an entity subclass may be defined as secondary keys, and such
secondary keys can only be used to query instances of the subclass. For
example, although the primary key (id) and secondary key (name) can be used to retrieve any Pet instance, the entity subclass
Cat defines a secondary key (finickyness) that only applies
to Cat instances. Querying by this key will never retrieve a Dog instance, if such a subclass existed, because a Dog instance
will never contain a finickyness key.

WARNING: Entity subclasses that define secondary keys must be
registered prior to storing an instance of the class. This can be done in
two ways:

The registerClass method may be called
to register the subclass before opening the entity store.

The getSubclassIndex method may be
called to implicitly register the subclass after opening the entity
store.

Persistent Fields and Types

All non-transient instance fields of an entity class, as well as its
superclasses and subclasses, are persistent. static and transient fields are not persistent. The persistent fields of a class may
be private, package-private (default access), protected or
public.

It is worthwhile to note the reasons that object persistence is defined
in terms of fields rather than properties (getters and setters). This
allows business methods (getters and setters) to be defined independently of
the persistent state of an object; for example, a setter method may perform
validation that could not be performed if it were called during object
deserialization. Similarly, this allows public methods to evolve somewhat
independently of the (typically non-public) persistent fields.

Persistent types are divided into simple types, enum types, complex
types, and array types. Simple types and enum types are single valued,
while array types may contain multiple elements and complex types may
contain one or more named fields.

Complex persistent classes must be annotated with Entity or
Persistent, or must be proxied by a persistent proxy class
(described below). This includes entity classes, subclasses and
superclasses, and all other complex classes referenced via fields of these
classes.

All complex persistent classes must have a default constructor. The
default constructor may be private, package-private (default
access), protected, or public. Other constructors are
allowed but are not used by the persistence mechanism.

It is sometimes desirable to store instances of a type that is externally
defined and cannot be annotated or does not have a default constructor; for
example, a class defined in the Java standard libraries or a 3rd party
library. In this case, a PersistentProxy class may be used to
represent the stored values for the externally defined type. The proxy
class itself must be annotated with Persistent like other persistent
classes, and the Persistent.proxyFor() property must be specified.

For convenience, built-in proxy classes are included for several common
classes (listed below) in the Java library. If you wish, you may define
your own PersistentProxy to override these built-in proxies.

HashSet

TreeSet

HashMap

TreeMap

ArrayList

LinkedList

Complex persistent types should in general be application-defined
classes. This gives the application control over the persistent state and
its evolution over time.

Other Type Restrictions

Entity classes and subclasses may not be used in field declarations for
persistent types. Fields of entity classes and subclasses must be simple
types or non-entity persistent types (annotated with Persistent not
with Entity).

Entity classes, subclasses and superclasses may be abstract and
may implement arbitrary interfaces. Interfaces do not need to be annotated
with Persistent in order to be used in a persistent class, since
interfaces do not contain instance fields.

Persistent instances of static nested classes are allowed, but the nested
class must be annotated with Persistent or Entity. Inner
classes (non-static nested classes, including anonymous classes) are not
currently allowed as persistent types.

Arrays of simple and persistent complex types are allowed as fields of
persistent types. Arrays may be multidimensional. However, an array may
not be stored as a top level instance in a primary index. Only instances of
entity classes and subclasses may be top level instances in a primary
index.

Embedded Objects

As stated above, the embedded (or member) non-transient non-static fields
of an entity class are themselves persistent and are stored along with their
parent entity object. This allows embedded objects to be stored in an
entity to an arbitrary depth.

There is no arbitrary limit to the nesting depth of embedded objects
within an entity; however, there is a practical limit. When an entity is
marshalled, each level of nesting is implemented internally via recursive
method calls. If the nesting depth is large enough, a StackOverflowError can occur. In practice, this has been observed with a
nesting depth of 12,000, using the default Java stack size.

This restriction on the nesting depth of embedded objects does not apply
to cyclic references, since these are handled specially as described
below.

Object Graphs

When an entity instance is stored, the graph of objects referenced via
its fields is stored and retrieved as a graph. In other words, if a single
instance is referenced by two or more fields when the entity is stored, the
same will be true when the entity is retrieved.

When a reference to a particular object is stored as a member field
inside that object or one of its embedded objects, this is called a cyclic
reference. Because multiple references to a single object are stored as
such, cycles are also represented correctly and do not cause infinite
recursion or infinite processing loops. If an entity containing a cyclic
reference is stored, the cyclic reference will be present when the entity is
retrieved.

Note that the stored object graph is restricted in scope to a single
entity instance. This is because each entity instance is stored separately.
If two entities have a reference to the same object when stored, they will
refer to two separate instances when the entities are retrieved.

Optional Element Summary

Identifies a new version of a class when an incompatible class change
has been made.

Element Detail

version

public abstract int version

Identifies a new version of a class when an incompatible class change
has been made. Prior versions of a class are referred to by version
number to perform class evolution and conversion using Mutations.

The first version of a class is version zero, if version() is
not specified. When an incompatible class change is made, a version
number must be assigned using version() that is higher than the
previous version number for the class. If this is not done, an IncompatibleClassException will be thrown when the store is opened.